"Don't listen to the wicked," we are cautioned. "Don't take the path that sinners tread."
Rather, the wise are supposed to pay attention to what God wants.
And according to this psalm, doing what God wants can make somebody happy. On the other hand, listening to what people who are wicked or sinful or plain lazy will have bad results.
I am struck by the imagery of the tree planted beside streams of water. I grew up in a place and a time with little rain. Driving through the countryside, we could easily identify where the creeks were still running because we could see a strand of trees in the middle of a field. A well-watered tree thrives, yielding fruit and shade.
Let us allow ourselves happiness by following the teaching of the Lord. Let us yield fruit and shade.